1. Field of the Invention
The present invention relates to a sheet material cutting apparatus for preventing a drive device which overload is mounted on the cutter of the cutting apparatus, with the cutting apparatus being used for cutting sheet materials such as, for example, recording paper and films in, for example, facsimile equipment and copying machines.
2. Description of the Prior Art
Conventional sheet cutting apparatuses, composed of reciprocating movable blades and fixed blades, are powered by motors exclusively provided for such purpose or main motors of the machines in order to transmit the power to the movable blades after the rotational movement of the motor is converted into a reciprocating movement. In such a conventional cutting machine, when jamming (sheet jamming) occurs at the cutter of the cutting apparatus, or when metal pieces or the like are engaged with the cutter edge, the cutter edge and parts in a driving system may be damaged when driving force is powerful or the cutter might stop during the operation when driving force is insufficient. In the former case, the cutting apparatus cannot be reused until a cutter with the damaged edge has been replaced. In the latter case, the motor may overheat, and the cutter must be manually returned to a waiting state in order to remove the sheets jammed in the cutter.
In order to prevent such problems from taking place, a torque limiter is mounted on the driving side of the cutting apparatus, or the current of the motor is controlled so as to limit torque transmission. In a system which converts rotational movement into reciprocating movement, however, it is difficult to convert fixed rotational torque into fixed reciprocating torque. The system is not sufficient for inhibiting the torque on the cutter side to less than a fixed amount, since a very high power effect is normally developed in the vicinities of upper and lower dead points.
In order to operate such a system in a more perfect manner, it is therefore desirable to inhibit power transmission at levels less than a fixed level after the rotational movement has been converted into reciprocating movement.
In, for example, Japanese Utility Model Application No. 61-34745 (Japanese Utility Model Unexamined Publication No. 62-147491), a system is proposed utilizing a quadric link system for transmitting rotational movement of a cam to a rotating blade in the form of reciprocating movement by a supporting lever and a spring. For a load exceeding a predetermined amount, the spring is expanded to inhibit the transmission of excessive power.
A disadvantage of the above-noted proposed system resides in the fact that, as is well known, springs expand at a fixed ratio under load and it is impossible to predict any position in a cutting stroke when overload may occur at the reciprocating movable blade of the cutter. Consequently, if overload occurs due to jamming at a position in which the cutter has just started cutting, the above-constructed cutter system is rotated around a driving or motor shaft while the spring is being expanded to a predetermined length. As a result, in addition to the overload generated in the position in which the cutter has just started cutting, the load generated by the expansion of the spring during cutting is transmitted.
On the other hand, if overload occurs at a position in which the cutter is approaching the finish of a cut, the cutter system operates to a limit substantially equal to the power at the occurrence of the overload, since the spring expands a little after the overload has occurred.
In other words, in this system, the magnitude of load that can be inhibited varies in dependence upon the position at which overload occurs, which fact is an obstacle to minimizing power requirements and the driving system.
In this system, if load alternations to be inhibited are decreased, it is effective to reduce the spring constant. If the spring constant is reduced while maintaining predetermined tension, a large-sized spring is required, which results in increased costs and installation space.